a) Field of the Invention
The present invention relate to copolymers of butene-1 and propylene and also to heat-sealable polypropylene resin compositions containing one or more of the copolymers and useful as heat-sealing layers for heat-bonding polyolefins.
b) Description of the Related Art
The existence of .alpha.-olefin polymers having a syndiotactic structure has been known for many years, including syndiotactic polypropylene by way of example. Syndiotactic polypropylene can be obtained by conducting polymerization at a low temperature in the presence a conventional catalyst composed of a vanadium compound, ether and an organoaluminum compound. This polypropylene however has poor syndiotacticity so that it can hardly be considered to have the properties of a syndiotactic polypropylene.
J. A. Ewen et al. has found for the first time that polypropylene having such good tacticity as exceeding 0.8 in terms of syndiotactic pentad fraction as measured by .sup.13 C-NMR can be obtained by polymerizing propylene in the presence of a polymerization catalyst composed of methyl aluminoxane and a transition metal (Hf and Zr) compound having an asymmetric ligand (J. Amer. Chem. Soc., 110, 6255-6, 1988).
In the meantime, it has been found by the present inventors that polybutene-1 having high syndiotacticity can be obtained when butene-1 is polymerized using the above catalyst in a high-purity form. This catalyst has good activity per transition metal and, moreover, the resultant polybutene-1 has high tacticity. However, the balancing of physical properties of the polybutene-1 is rather poor and articles molded from the polybutene-1 have insufficient transparency.
Polypropylene resins which substantially have stereoregularity, namely, either an isotactic structure or a syndiotactic structure have excellent stiffness and are superior in external appearance such as transparency and gloss. They are hence used for various applications. For films, propylene-ethylene copolymers are used to improve impact resistance and heat sealability. As resins having excellent balance among stiffness, impact resistance and heat sealability, propylene-ethylene-butene-1 terpolymers are known by way of example.
Although these copolymers have excellent balance among stiffness, impact resistance and heat sealability, it is desired to improve the heat sealability further so that the efficiency in the use of films can be improved.